Method for regulating an electrolysis cell

ABSTRACT

The invention relates to a regulation method for an electrolytic cell for the production of aluminium by means of reduction of alumina dissolved in a molten cryolite bath and comprises the addition, in the electrolyte bath, during pre-determined time intervals p referred to as “periods”, of a determined quantity Q(p) of aluminium trifluoride (AlF 3 ) determined by the following equation: Q(p)=Qint(p)−Qc1(p)+Qt(p), where Qint(p) is an integral (or “self-adaptive”) term which represents the total actual AlF 3  requirements of the cell and which is calculated from a mean Qm(p) of the actual AlF 3  supplies made during the last N periods, Qc1 is a compensating term corresponding to the so-called “equivalent” quantity of AlF 3  contained in the alumina added to the cell during the period p, and Qt(p) is a corrective term which is a typically increasing function of the difference between the measured bath temperature T(p) and the set-point temperature To. The method according to the invention makes it possible to regulate effectively the acidity of an electrolytic cell at intensities of up to 500 kA with an electrolyte bath having an AlF 3  content greater than 11%.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage Application ofPCT/FR02/00705 filed Feb. 27, 2002, which claims priority to FrenchApplication No. 01/02722 which was filed Feb. 28, 2001.

1. Field of the Invention

The invention relates to a regulation method for an aluminium productioncell by means of electrolysis of alumina dissolved in an electrolytebased on molten cryolite, particularly according to the Hall-Héroultmethod. It particularly relates to the regulation of the quantity ofaluminium trifluoride (AlF₃) of the cryolite bath.

2. State of the Art

Metal aluminium is produced industrially by igneous electrolysis, i.e.by means of electrolysis of alumina in solution in a molten cryolitebath, referred to as an electrolyte bath, particularly according to thewell-known Hall-Héroult method. The electrolyte bath is contained inpots, referred to as “electrolytic pots”, comprising a steel shell,which is coated internally with refractory and/or insulating materials,and a cathode assembly located at the base of the pot. Anodes made ofcarbonaceous materials are partially immersed in the electrolyte bath.The assembly formed by an electrolytic pot, its anode(s) and theelectrolyte bath is referred to as an electrolytic cell.

The electrolytic current, which flows in the electrolyte bath and thepad of liquid aluminium via the anodes and cathode components, bringsabout the aluminium reduction reactions and also makes it possible tomaintain the electrolyte bath at a temperature of the order of 950° C.by means of the Joule effect. The electrolytic cell is regularlysupplied with alumina so as to compensate for the alumina consumptionproduced by the electrolytic reactions.

The productivity and Current efficiency of an electrolytic cell areinfluenced by several factors, such as the intensity and distribution ofthe electrolytic current, the pot temperature, the dissolved aluminacontent and the acidity of the electrolyte bath, etc., which interactwith each other. For example, the melting temperature of a cryolite bathdecreases with the excess aluminium trifluoride (AlF₃) with reference tothe nominal composition (3 NaF.AlF₃). In modern plants, the operatingparameters are adjusted to aim for Current efficiencies of over 90%.

However, the effective Current efficiency of a cell is significantlyinfluenced by variations in said cell's parameters. For example, anincrease in the electrolyte temperature by around ten degrees Celsiusmay cause the Current efficiency to fall by approximately 2% and adecrease in the electrolyte temperature by around ten degrees Celsiusmay reduce the already low solubility of alumina in the electrolyte andfavour the “anode effect”, i.e. anode polarisation, with a sudden risein the voltage at the cell terminals and the release of a large quantityof fluorinated and fluoro-carbonated products, and/or insulatingdeposits on the cathode surface.

Therefore, the operation of an electrolytic cell requires precisecontrol of its operating parameters, such as its temperature, aluminacontent, acidity, etc., so as to maintain them at determined set-pointvalues. Several regulation methods have been developed to achieve thisobjective. These methods generally relate to the regulation of thealumina content of the alumina bath, the regulation of its temperature,or the regulation of its acidity, i.e. the excess AlF₃.

The American patent U.S. Pat. No. 4,668,350 discloses a method tocontrol AlF₃ additions wherein AlF₃ is added at a determined rate, thetemperature of the bath is measured regularly and the AlF₃ addition rateis adjusted according to the difference between the temperature measuredin the pot and the target temperature (the addition rate is increasedwhen the temperature measured is greater than the set-point temperatureand decreased otherwise). The AlF₃ addition rate can also be correctedaccording to the deviation of the temperature measured (the rate isincreased when the temperature measured is greater than the previousvalue and decreased otherwise). This method, which is based on thecorrelation between the temperature and the AlF₃ content of the bath,does not take into account the impact of transient periods. In addition,this method handles thermal deviations poorly since it does not takeinto account the actual quantity of AlF₃ contained in the pot.

The American patent U.S. Pat. No. 5,094,728 discloses a regulationmethod wherein the optimal time lag between AlF₃ additions and theireffect on the electrolyte is calculated using a model comprising severalparameters, and the quantities of AlF₃ to be added during the next ndays are calculated using, firstly, the difference between the targetAlF₃ concentration of the bath and the measured value and, secondly, thetheoretical daily consumption. The parameters are calculated using themeasurements made on the pot during a long time interval, of the orderof 10 to 60 days. This method requires the development and set-up of acomplex model which is moreover not disclosed in this document.

The international application WO 99/41432 discloses a regulation methodwherein the liquidus temperature of the electrolyte bath is measured andthe liquidus temperature measured is compared to a first and a secondset-point value; if the liquidus temperature is greater than the firstset-point value, AlF₃ is added; if it is less than the second set-pointvalue, NaF or Na₂CO₃ is added. This regulation method requires areliable, rapid and economical measurement of the liquidus temperature.The liquidus temperature is generally determined from a complex equationwhich takes into account the exact composition of the electrolyte bath,particularly its NaF, AlF₃, CaF₂, LiF and Al₂O₃ contents.

Statement of the Problem

Aluminium producers, in the continuous aim to increase electrolyticplant production and productivity at the same time, try to push backthese limits.

In particular, in order to increase plant productivity, it is aimed toreach Current efficiencies above 95% operating with AlF₃ excesses ofover 11%, and which may reach 13 to 14%, which makes it possible todecrease the cell operating temperature (the liquidus temperature dropsapproximately 5° C./% AlF₃) and, as a result, reduce the energyconsumption of said cells. However, in this chemical composition range,the solubility of alumina is considerably reduced, which increases therisks of anode effects and of insulating deposits on the cathode.

In addition, in order to increase plant production, it is aimed toincrease the unit capacity of cells and, in correlation, increase theintensity of the electrolytic current. The current trend is to developelectrolytic cells with a current greater than or equal to 500 kA. Theincrease in the capacity of electrolytic cells may be obtained, as ageneral rule, either by increasing the permissible intensity of cells ofknown type or existing cells, or by developing very large cells. In thefirst case, the increase in the permissible intensity results in adecrease in the electrolyte bath mass, which exacerbates the instabilityeffect. In the second case, the increase in the cell size increasestheir thermal and chemical inertia. Consequently, the increase in cellcapacity not only increases the rate of alumina consumption but alsoamplifies instability generation and cell deviation phenomena, whichincreases difficulties in controlling electrolytic cells.

Therefore, the applicant searched for a regulation method for anelectrolytic cell, particularly of the electrolyte bath acidity (i.e.its AlF₃ content) and the overall thermics of the cell, which makes itpossible to control, in a stable manner with a Current efficiencygreater than 93%, or even greater than 95%, without having to usefrequent AlF₃ content measurements, electrolytic cells wherein theexcess AlF₃ is greater than 11% and wherein the current may be greaterthan or equal to 500 kA.

DESCRIPTION OF THE INVENTION

The invention relates to a regulation method for an electrolytic cellintended for the production of aluminium by means of igneouselectrolysis, i.e. by flowing current in an electrolyte bath based onmolten cryolite and containing dissolved alumina, particularly accordingto the Hall-Héroult method.

The regulation method according to the invention comprises the addition,in the electrolyte bath of an electrolytic cell, during pre-determinedtime intervals p referred to as “regulation periods”, of a determinedquantity Q(p) of aluminium trifluoride (AlF₃) determined by thefollowing equation:Q(p)=Qint(p)−Qc1(p)+Qt(p)

where

Qint(p) is an integral (or “self-adaptive”) term which represents thetotal actual AlF₃ requirements of the cell and which is calculated froma determination Qm(p) of the actual AlF₃ supplies made during the lastperiod or the last N periods, Qc1 is a compensating term correspondingto the so-called “equivalent” quantity of AlF₃ contained in the aluminaadded to the cell during the period p, said quantity being possiblypositive or negative,

Qt(p) is a corrective term which is a determined function (which istypically increasing) of the difference between the measured bathtemperature T(p) and the set-point temperature To.

The term Qint(p) takes into account AlF₃ losses in the bath occurringduring normal cell operation and which are essentially produced byabsorption by the pot crucible and emissions in gaseous effluents. Thisterm, the mean value of which is not equal to zero, is particularly usedto monitor pot ageing, without having to model it, by means of a memoryeffect of pot behaviour over time. It also takes into account thespecific ageing of each pot, that the applicant generally found to bemarkedly different to the average ageing of the population of pots ofthe same type.

The term Qm(p) takes into account total equivalent AlF₃ supplies, i.e.“direct” supplies from additions of AlF₃ and “indirect” supplies fromadditions of alumina.

In a preferred alternative embodiment of the invention, the calculationformula of the quantity Q(p) comprises an additional term Qc2(p), i.e.Q(p)=Qint(p)−Qc1(p)+Qt(p)+Qc2(p), where Qc2(p) is a corrective termwhich is a determined function (which is typically decreasing) of thedifference between Qm(p) and Qint(p).

The term Qc2 is a prospective correction term which is used to take intoaccount the effect of an addition of AlF₃ in advance, which normallyonly appears after a few days. Indeed, the applicant noted thesurprising degree of the difference between the time constant of thetemperature variation, which is low (of the order of a few hours) andthat of the AlF₃ content, which is very high (of the order of a few tensof hours). In its tests, it found that it was very advantageous toanticipate the variation of the acidity of the pot when adding AlF₃,which is made possible effectively by the term Qc2.

The terms Qt(p) and Qc2(p) are terms wherein the mean value over timenormally tends towards zero (i.e. they are normally equal to zero onaverage).

In addition, the applicant noted in its tests that the combined effectof the basic terms, i.e. Qt, Qint, Qc1 and, advantageously, Qc2, made itpossible to provide reliable regulation, i.e. with a high stability, ofthe AlF₃ content of electrolytic cells, over a period of several months,even without accounting for measured AlF₃ contents, which measurementsadd to cell operating costs and are, in any case, easily affected bysignificant errors.

FIGURES

FIG. 1 represents, in a transverse section, a typical electrolytic cell.

FIG. 2 illustrates the principle of the regulation sequences accordingto the invention.

FIG. 3 shows variations in the total AlF₃ requirements of anelectrolytic cell.

FIGS. 4 and 5 show typical functions used to determine the terms of Qtand Qc2.

FIG. 6 illustrates a method to determine the specific electricresistance variation of the electrolytic cell.

FIG. 7 is a schematic illustration of the shape of the current linesflowing in the electrolyte bath between an anode and the liquid metalpad.

FIG. 8 illustrates a method to determine the surface area of the liquidmetal pad.

As illustrated in FIG. 1, an electrolytic cell 1 for the production ofaluminium by means of the electrolysis method typically comprises a pot20, anodes 7 supported by attachment means 8, 9 to an anode frame 10 andalumina supply means 11. The pot 20 comprises a steel shell, internallining components 3, 4 and a cathode assembly 5, 6. The internal liningcomponents 3, 4 are generally blocks made of refractory materials, whichmay be heat insulators. The cathode assembly 5, 6 comprises connectionbars 6 to which the electric conductors used to route the electrolyticcurrent are attached.

The lining components 3, 4 and the cathode assembly 5, 6 form, insidethe pot 20, a crucible capable of containing the electrolyte bath 13 anda liquid metal pad 12 when the cell is in operation, during which theanodes 7 are partially immersed in the electrolyte bath 13. Theelectrolyte bath contains dissolved alumina and, as a general rule, analumina layer 14 covers the electrolyte bath.

The electrolytic current transits in the electrolyte bath 13 via theanode frame 10, the attachment means 8, 9, anodes 7 and cathodecomponents 5, 6. The purpose of the alumina supply to the cell is tocompensate for the approximately continuous consumption of the cellwhich is essentially due to the reduction of alumina into metalaluminium. The alumina supply, which is made by adding alumina into theliquid bath 13 (typically using an crustbreaker-feeder 11) is generallyregulated separately.

The metal aluminium 12 which is produced during the electrolysis isaccumulated at the bottom of the cell and a relatively sharp interfacebetween the liquid metal 6 and the molten cryolite bath 13 isestablished. The position of this bath-metal interface varies over time:it rises as the liquid metal accumulates at the bottom of the cell andit goes down when the liquid metal is removed from the cell.

As a general rule, an attempt is made to form a ridge 15 of solidifiedcryolite on the part of the side walls 3 of the crucible which are incontact with the electrolyte bath 7 and with the liquid metal pad 12.

Several electrolytic cells are generally arranged in a row, in buildingsreferred to as electrolysis rooms, and connected electrically in seriesusing connection conductors. The cells are typically arranged so as toform two or more parallel lines. The electrolytic current thus flows incascade from one cell to the next.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the regulation method for an electrolyticcell for the production of aluminium 1 by means of electrolyticreduction of alumina dissolved in an electrolyte bath 13 based oncryolite, said cell 1 comprising a pot 20, anodes 7 and cathodecomponents 5, 6 capable of circulating a so-called electrolytic currentin said bath, the aluminium produced by means of said reduction forminga pad referred to as a “liquid metal pad” 12 on said cathode components5, 6, said method comprising the supply of said cell with alumina insaid bath and being characterised in that it comprises:

the set-up of a regulation sequence comprising a series of timeintervals p of a duration Lp hereafter referred to as “regulationperiods” or simply “periods”;

the determination of a mean temperature T(p) of the electrolyte bath,from at least one measurement of the temperature of said bath madeduring the last period or at least one of the last Nt periods;

the determination of a so-called “equivalent” quantity Qc1(p) of AlF₃contained in the alumina added to the cell during the period p;

the determination of a value Qm(p) of the total equivalent AlF₃ suppliesper period during the last period or during the last N periods;

the determination of a quantity Q(p) of aluminium trifluoride (AlF₃) tobe added during the period p, referred to as “determined quantity Q(p)”,using the formula:Q(p)=Qint(p)−Qc1(p)+Qt(p),

where Qint(p)=α×Qm(p)+(1−α)×Qint(p−1),

α is a smoothing coefficient setting the temporal smoothing horizon ofthe integral term Qint(p),

Qt(p) is a determined function, preferentially increasing, of thedifference between said temperature T(p) and a set-point temperature To,

the addition in said electrolyte bath, during the period p, of aneffective quantity of aluminium fluoride (AlF₃) equal to said determinedquantity Q(p).

The term Q(p) corresponds to an addition of pure AlF₃ and is typicallyexpressed in kg of pure AlF₃ per period (kg/period). The expression“addition of an effective quantity of AlF₃” corresponds to an additionof pure AlF₃. In industrial practice, AlF₃ additions are generally madeusing so-called industrial AlF₃ with a purity of less than 100%(typically 90%). In this case, a sufficient quantity of industrial AlF₃is added to obtain the effective quantity of AlF₃ required. Typically, aquantity of industrial AlF₃ equal to the effective quantity of AlF₃required divided by the purity of the industrial AlF₃ used is added.

The expression “total AlF₃ additions” refers to the sum of the effectiveadditions of pure AlF₃ and the “equivalent” AlF₃ additions from alumina.

AlF₃ may be added in different ways. It may be added manually ormechanically (preferentially using a point feed such as ancrustbreaker-feeder which makes it possible to add determined doses ofAlF₃, in an automated fashion if required). AlF₃ may be added withalumina or at the same time as alumina.

The different terms of Q are determined preferentially at each period p.If the cell is very stable, it may be sufficient to determine thequantity Q(p) and some of the terms forming it, in a more staggeredmanner over time, for example once every two or three periods.

The quantity Q(p) is normally determined at each period. If one or moreterms of Q(p) cannot be calculated during a given period, then it ispossible to maintain the value of said term(s) used during the previousperiod, i.e. the value of said term(s) will be determined by making itequal to the value used during the last period. If one or more termscannot be calculated during several periods, then it is possible toretain the value of said term(s) used during the previous period forwhich it could be calculated and maintain this value for a limitednumber Ns of periods (Ns being typically equal to 2 or 3). In the lattercase, if said term(s) still cannot be calculated after the Ns periods,then it is possible to retain the pre-determined fixed value, referredto as the “standby value”. These different situations may occur, forexample, when the mean temperature of the pot cannot be determined orwhen the equivalent AlF₃ quantity contained in the alumina could not bedetermined.

The intervals (or “periods”) p are preferentially approximately equal inlength Lp, i.e. the length Lp of the periods is approximately the samefor all the periods, enabling easier implementation of the invention.Said length Lp is generally between 1 and 100 hours.

As shown in FIG. 2, the additions of AlF₃ may be made at any time duringsaid regulation periods (or sequences), which may correspond to the workshifts which determine the frequency of the changes of the shifts incharge of cell control and maintenance. The quantity Q(p) of AlF₃determined for a period p may be added in one or more times during saidworking period. Preferentially, the quantity Q(p) is added practicallycontinuously using crustbreaker-feeders which make it possible to addpredetermined doses of AlF₃ throughout the period p.

In a preferred alternative embodiment of the invention, the term Qm(p)is calculated using the equation:Qm(p)=<Q(p)>+<Qc1(p)>, where

<Q(p)>=Q(p−1) and <Qc1(p)>=Qc1(p−1) when the term Qm(p) is determinedusing the total equivalent AlF₃ supplies during the last period, i.e.p−1;

<Q(P)>=(Q(p−N)+Q(p−N+1)+ . . . +Q(p−1))/N, and<Qc1(p)>=(Qc1(p−N)+Qc1(p−N+1)+ . . . +Qc1(p−1))/N, when the term Qm(p)is determined using total equivalent AlF₃ supplies during the last Nperiods, i.e. p−1, p−2, . . . , N.

Therefore, in the latter case, the term Qm(p) is equal to(Q(p−2)+Qc1(p−2)+Q(p−1)+Qc1(p−1))/2 when N=2;(Q(p−3)+Qc1(p−3)+(Q(p−2)+Qc1(p−2)+Q(p−1)+Qc1(p−1))/3 when N=3, . . .

The value of the parameter N is selected according to the cell reactiontime and is normally between 2 and 100, and more typically between 2 and20.

In order to converge the integral term Qint(p) rapidly to the quantityQ′ corresponding to actual cell requirements, it is possible to startthe method by simply taking Qint(0)=Qtheo, where Qtheo corresponds tothe total theoretical AlF₃ requirements of the cell when regulation isstarted. Qtheo is a function of the age of the pot which can bedetermined statistically for each type of pot.

This alternative embodiment may be implemented by including in themethod according to the invention:

the determination of a quantity Qtheo corresponding to the totaltheoretical AlF₃ requirements of the cell when regulation is started;

the start-up of the method by taking Qint(0)=Qtheo.

The smoothing coefficient α, which makes it possible to do away withmedium and long-term thermal and chemical fluctuations, is equal toLp/Pc, where Pc is a period which is typically of the order of 400 to8000 hours, and more typically of 600 to 4500 hours, and Lp is thelength of one period. Therefore, the term 1/α is typically equal to 50to 1000 8-hour periods if this work organisation mode is applied.

The term Qc1(p) is determined by producing the chemical balance of thefluorine and sodium contained in said alumina from one or more chemicalanalyses. The effect of the sodium contained in the alumina is toneutralise fluorine, then amounting to a negative quantity of AlF₃. Theterm Q1c(p) is positive if said alumina is “fluorinated” (which is thecase when it has been used to filter electrolytic cell effluents) andnegative if the alumina is “fresh”, i.e. if it is produced directly fromthe Bayer process.

The regulation term Qt(p) is given by a determined function (typicallyincreasing and preferentially limited by a maximum value and a minimumvalue) of the difference between the measured temperature of the bathT(p) and a set-point temperature To. FIG. 4 shows a typical functionused to determine the term Qt.

This alternative embodiment may be implemented by including in themethod according to the invention:

the determination of a mean temperature T(p) of the electrolyte bath;

the determination of the term Qt(p) using a determined function (whichis typically increasing and preferentially limited) of the differencebetween said temperature T(p) and the set-point temperature To.

In a simplified alternative embodiment of the invention, the term Qt(p)may follow a simple equation, such as Qt(p)=Kt×(T(p)−To), where Kt is aconstant which is typically positive and which may be set empiricallyand wherein the value is typically between 0.01 and 1 kg/hour/° C., andmore typically between 0.1 and 0.3 kg/hour/° C. (corresponding, in thelatter case, to approximately 1 to 2 kg/period/° C. for 8-hour periods)for 300 kA to 500 kA pots.

The term Qt(p) is preferentially limited by a minimum value and by amaximum value.

The mean temperature T(p) is normally determined from temperaturemeasurements made on the period p and on the previous periods p−1, etc.,so as to obtain a reliable and significant value of the averagecondition of the pot.

The term Qc2(p) is given by a determined function (which is typicallydecreasing and preferentially limited) of the difference Qm(p)−Qint(p).This damping term takes into account the delay in the reaction of thecell with the AlF₃ additions. FIG. 5 shows a typical function used todetermine the term Qc2.

In a simplified alternative embodiment of the invention, the term Qc2(p)may follow a simple equation, such as Qc2(p)=Ko2×(Qm(p)−Qint(p)), whereKo2 is a constant which is typically negative and which may be setempirically and wherein the value is typically between −0.1 and −1, andmore typically between −0.5 and −1 for 300 kA to 500 kA pots.

The term Qc2(p) is preferentially limited by a minimum value and by amaximum value.

In an advantageous alternative embodiment of the method according to theinvention, the quantity Q(p) comprises an additional regulation term,Qr(p), which is sensitive to the thickness (and, to a lesser extent, theshape) of the solidified bath ridge 15 formed on the walls 3 of the cell1 via the spreading η of the lines of current in the electrolyte bath.

This term may particularly be used when the electrolytic cell comprisesa mobile anode frame 10 to which the anodes 7 of the cell are attachedand means (not shown) to move said anode frame 10. As shown in FIG. 6,said resistance is typically measured using means 18 to measure theintensity Io of the current circulating in the cell (where Io is equalto the sum of the cathode currents Ic or anode current Ia) and means 16,17 to measure the resulting drop in voltage U at the cell terminals (andmore specifically the resulting drop in voltage between the anode frameand the cathode components of the cell). Said resistance is generallycalculated using the equation: R=(U−Uo)/Io, where Uo is a constanttypically between 1.6 and 2.0 V.

The term Qr(p) is given by a determined function (which is typicallydecreasing and preferentially limited) of a quantity referred to as“specific resistance variation” ΔRS which is equal to ΔR/ΔH, where ΔR isthe variation of the resistance R at the terminals of the electrolyticcell when the anode frame 10 is moved by a determined distance ΔH,either upwards (AH positive), or downwards (ΔH negative). In practice,it was found to be simpler to give an order of movement of the anodeframe 10 for a determined time and measure the resulting frame movementΔH. The term Qr(p) is advantageously a function of the differencebetween ΔRS and a reference value ΔRSo.

According to this alternative embodiment of the invention, the methodadvantageously comprises:

the movement of the anode frame 10 by a determined distance ΔH, eitherupwards (ΔH being positive in this case), or downwards (ΔH beingnegative in this case);

the measurement of the variation ΔR of the resistance R resulting fromsaid movement;

the calculation of a specific resistance variation ΔRS using theformula:ΔRS=ΔR/ΔH;

the determination of a term Qr(p) using a determined function (which istypically decreasing) of the specific resistance variation ΔRS;

the addition of the term Qr(p) in the determination of the quantityQ(p).

The resistance R depends not only on the resistivity p of theelectrolyte bath 13, on the distance H between the anode(s) 7 and theliquid metal pad 12, and on the surface area Sa of the anode(s) 7, butalso on the spreading η of the lines of current Jc, Js which areestablished in said bath, particularly between the anode(s) 7 and thesolidified bath ridge 15 (lines Jc in FIG. 7). The applicant had theidea to make use of the fact that the specific electric resistancevariation ΔRS is not only sensitive to the resistivity of theelectrolyte bath, but integrates an electric current spreading factor,which is sensitive to the presence, size and, to a lesser degree, shapeof the solidified bath ridge 15 on the walls of the pot 20.

The applicant also observed that, unlike that which is normallyadmitted, the spreading η is in fact a preponderant factor in theestablishment of electric resistance. The applicant considers that thecontribution of spreading to the specific electric resistance variationis typically between 75 and 90%, which means that the contribution ofthe resistivity is very low, that is typically between 10 and 25%(typically 15%). In its tests on 500 kA pots, the applicant observed amean ΔRS value of the order of 100 mΩ/mm, which decreases byapproximately −3 nΩ/mm when the bath temperature increases by 5° C. andwhen the AlF₃ content decreases by 1%, and conversely. The contributionof the resistivity to this variation is estimated to be only of theorder of −0.5 nΩ/mm (that is only approximately 15% of the total value),the contribution attributable to spreading, i.e. −2.5 nΩ/mm beingdominant.

It is possible to take into account the spreading of the current in theresistance measured (for example by modelling the current lines), whichconsiderably improves the reliability of the corrective term Qr(p) as anindicator of the thermal state of the cell.

In a simplified alternative embodiment of the invention, the term Qr(p)may be given by a simple equation such as: Qr(p)=Kr×(ΔRS−ΔRSo), where Kris a constant which may be set empirically and wherein the value istypically between −0.01 and −10 kg/hour/nΩ/mm, and more typicallybetween −0.05 and −0.3 kg/hour/nΩ/mm (corresponding, in the latter case,to approximately −0.5 to −2 kg/period/nΩ/mm for 8-hour periods) for 300kA to 500 kA pots.

The term Qr(p) is preferentially limited by a minimum value and by amaximum value.

In practice, it is possible to make Nr measurements of ΔRS (i.e. two ormore measurements) during the period p. The ΔRS value used to calculateQr(p) will in this case be the mean of the Nr measured ΔRS values,except, if applicable, values considered to be aberrant. It is alsopossible to use a sliding mean on two or more periods to smooth thethermal fluctuations related to the operating cycle. An operating cycleis determined by the frequency of interventions on the electrolyticcell, particularly anode replacements and liquid metal sampling. Thelength of an operating cycle is generally between 24 and 48 hours (forexample 4×8-hour periods).

In another advantageous alternative embodiment of the method accordingto the invention, the quantity Q(p) comprises an additional regulationterm, Qs(p), which is given by a determined function (which is typicallyincreasing and preferentially limited) of the difference between thesurface area S(p) of the liquid metal pad 12 and a set-point value So.

According to this alternative embodiment of the invention, the methodadvantageously comprises:

the determination of a term Qs(p);

the addition of the term Qs(p) in the determination of the quantityQ(p).

The surface area S(p), which corresponds approximately to the metal/bathinterface, is approximately equal to the horizontal right section of theelectrolytic pot. The presence of solidified electrolyte bath on thewalls of the pot decreases this surface area by a quantity which variesas a function of time and pot operating conditions.

The term Qs(p) is given by a determined function (which is typicallyincreasing and preferentially limited) of the difference S(p)−So. In asimplified alternative embodiment of the invention, the term Qs(p) maybe given by a simple equation such as: Qs(p)=Ks×(S(p)−So), where Ks is aconstant which may be set empirically and wherein the value is typicallybetween 0.0001 and 0.1 kg/hour/dm², and more typically between 0.001 and0.01 kg/hour/dm² (corresponding, in the latter case, to approximately0.01 to 0.05 kg/period/dm² for 8-hour periods) for 300 kA to 500 kApots.

The term Qs(p) is preferentially limited by a minimum value and by amaximum value.

In the preferred embodiment of this alternative embodiment of theinvention, the surface area S(p) is calculated from a measurement of thevolume Vm of metal tapped and the fall ΔHm of the corresponding metallevel Hm (see FIG. 8). More specifically, the volume Vm of liquid metalextracted from the electrolytic pot is measured (typically with ameasurement of this metal mass) and the change ΔHm of the resultingliquid metal level, and the surface area S(p) is then calculated usingthe equation S(p)=Vm/ΔHm. In practice, in order to maintain themetal/anode distance constant, the anodes 9 are normally lowered at thesame time as the liquid metal level.

The applicant noted that the corrective terms Qr(p) and Qs(p) accordingto the present application are effective indicators of the overallthermal state of the electrolytic cell, which take into account both theliquid electrolyte bath and the solidified bath ridge on the walls ofthe pot. These terms, taken separately or in combination, particularlymake it possible to reduce the number of analyses of the AlF₃ content inthe liquid electrolyte bath markedly and thus complete the correctionmade by the term Qt(p). The applicant observed that the frequency of theanalyses of the AlF₃ content may be reduced typically to one analysisper cell approximately every 30 days. The terms Qr(p) and Qs(p) make itpossible to only perform AlF₃ content analyses in exceptional cases orin order to characterise a cell or a series of cells statistically.

In another advantageous alternative embodiment of the invention, thequantity Q(p) comprises an additional corrective term Qe(p) which is adetermined function (which is typically decreasing and preferentiallylimited) of the difference between the excess AlF₃ measured E(p) and itstarget value Eo, i.e. the difference E(p)−Eo.

This alternative embodiment may be implemented by including in themethod according to the invention:

the measurements of the excess AlF₃ E(p);

the determination of an additional corrective term Qe(p) using adetermined function (typically decreasing and preferentially limited) ofthe difference between the excess AlF₃ measured E(p) and its targetvalue Eo, i.e. the difference E(p)−Eo;

the determination of the quantity Q(p) by adding the term Qe(p) in thecalculation.

In a simplified alternative embodiment of the invention, the term Qe(p)may be given by a simple equation such as: Qe(p)=Ke×(E(p)−Eo), where Keis a constant which may be set empirically and wherein the value istypically between −0.05 and −5 kg/hour/% AlF₃, and more typicallybetween −0.5 and −3 kg/hour/% AlF₃ (corresponding, in the latter case,to approximately −20 to −5 kg/period/% AlF₃ for 8-hour periods) for 300kA to 500 kA pots.

The term Qe(p) is preferentially limited by a minimum value and by amaximum value.

The applicant found it was satisfactory to only apply the term Qe(p)exceptionally, for a short length of time, when the thermal operation ofthe cell leaves the normal operating range, i.e. when the indicators(such as the temperature, ΔRS, S, etc.) leave the so-called safetyranges.

The applicant noted in its tests that the corrective term Qe enabled theindicators (temperature, ΔRS, S, etc.) to return rapidly to the normaloperating range.

According to another alternative embodiment of the invention, it is alsopossible to add corrective terms to take into account individualinterfering events.

In particular, the regulation may comprise a so-called anode effect termQea to take into account the impact of an anode effect on the thermicsof an electrolytic cell. An anode effect particularly inducessignificant AlF₃ losses by emission and, generally, heating of theelectrolyte bath. The term Qea is applied for a limited time followingthe observation of an anode effect. The term Qea is calculated usingeither a scale which is a function of the anode effect energy (AEE), ora fixed mean value. In the first case, the term Qea is given by adetermined function (which is typically increasing and preferentiallylimited) of the energy AEE. The term Qea(p) is preferentially limited bya minimum value and by a maximum value.

Industrial bath and pure cryolite additions are sometimes performed onindustrial cells. These additions have an impact on the composition ofthe electrolyte bath which must generally be taken into account in theregulation. For this purpose, the regulation method may also comprise acorrective term Qb to take into account the modification of the pureAlF₃ content induced by these additions.

In order to prevent excess AlF₃ additions, it is preferable, as aprecaution, to limit Q(p) to a maximum value Qmax. It is also preferableto limit the application of the regulation terms in time when theycannot be determined at each period.

The applicant observed that it was sufficient to only apply some of theterms of Q(p), such as Qe(p), exceptionally and for a limited length oftime, which makes it possible to limit costs relating to theirdetermination.

The term Q(p) may be positive, null or negative. In the last case, it isassumed that Q(p)=0, i.e. AlF₃ is not added during the period p. Whenthe term Q(p) is negative, it is also possible to correct thecomposition of the electrolyte bath 13 by adding soda, i.e. calcinedsoda or sodium carbonate, referred to as soda ash.

EXAMPLES OF EMBODIMENTS OF THE INVENTION

The following examples illustrate the calculations inherent to theregulation method according to the invention. These calculations aretypical of those made for the 500 kA cells tested by the applicant. Thelength of the periods is 8 hours.

Example 1

Example illustrating the use of the basic terms Qint, Qc1, Qc2 and Qtfor a pot of average age (28 months).

The value of Qtheo at 28 months is +31 kg/period. The averagerequirements of the cell Q′ determined by the integral term Qint are +39kg/period.

The alumina analysis gives a value of 1.36% of fluorine and 5250 ppm ofNa₂O equivalent. The alumina consumption of the cell during one 8-hourperiod is 2400 kg. The term Qc1 is then equal to +22 kg/period inequivalent pure AlF₃ supply.

By taking N=12, the total actual AlF₃ supplies per period over the lastN periods is 44 kg/period. The difference between the actual supplies(44 kg/period) and the mean requirements (39 kg/period) is then +5kg/period. The term Qc2 is then equal to −3 kg/period.

The temperature measured is 957° C. and the set-point temperature 953°C., i.e. a difference of +4° C. The corrective term Qt is then equal to+7 kg/period.

The quantity of AlF₃ to be added during the period p is then equal to:Q(p)=Qint(p)−Qc1(p)+Qc2(p)+Qt(p)=39−22−3+7=+21 kg.

Example 2

Example illustrating the use of the basic terms Qint, Qc1, Qc2 and Qtfor a young pot (7 months).

The value of Qtheo at 7 months is +23 kg/period. The averagerequirements of the cell Q′ determined by the integral term Qint are +32kg/period. The term Qc1 is equal to +20 kg/period in equivalent pureAlF₃ supply. The term Qc2 is equal to −6 kg/period.

The temperature measured is 964.6° C. and the set-point temperature 956°C., i.e. a difference of +8.6° C. The corrective term Qt is then equalto +15 kg/period.

The quantity of AlF₃ to be added during the period p is then equal to:Q(p)=Qint(p)−Qc1(p)+Qc2(p)+Qt(p)=32−20−6+15=+21 kg.

Example 3

Example illustrating the use of the basic terms Qint, Qc1, Qc2 and Qt,corrected with term Qe for a young cell (6 months).

The value of Qtheo at 7 months is +23 kg/period. The averagerequirements of the cell Q′ determined by the integral term Qint are +32kg/period. The term Qc1 is equal to +20 kg/period in equivalent pureAlF₃ supply. The term Qc2 is equal to −6 kg/period. The corrective termQt is equal to +15 kg/period.

The AlF₃ rate measured is 12.8% and the set-point value is 12.0%. Thevalue of Qe is then −14 kg/period.

The quantity of AlF₃ to be added during the period p is then equal to:Q(p)=Qint(p)−Qc1(p)+Qc2(p)+Qt(p)+Qe(p)=32−20−6+15−14=+7 kg. The term Qethus prevents an over-correction of the AlF₃ content.

Example 4

Example illustrating the use of the additional terms Qr and Qs incombination with the basic terms Qint, Qc1, Qc2 and Qt.

The value of Qtheo at 28 months is +31 kg/period. The averagerequirements of the cell Q′ determined by the integral term Qint are +39kg/period. The term Qc1 is equal to +22 kg/period in equivalent pureAlF₃ supply. The term Qc2 is equal to −3 kg/period.

The temperature measured is 964° C. and the set-point temperature 953°C., i.e. a difference of +10.8° C. The corrective term Qt is then equalto +18 kg/period.

The ΔRS value measured is 101.8 nΩ/mm and the set-point value ΔRSo is106.0 nΩ/mm. The term Qr(p) is then equal to +5 kg/period.

The S value measured is 6985 dm² and the set-point value So is 6700 dm².The term Qs(p) is then equal to +5 kg/period.

The quantity of AlF₃ to be added during the period p is then equal to:Q(p)=Qint(p)−Qc1(p)+Qc2(p)+Qt(p)+Qr(p)+Qs(p)=39−22−3+18+5+5=+42 kg. Theterms Qr and Qs make a significant correction to the quantity Q(p).

Tests

The method according to the invention was used to regulate electrolyticcells with intensities of up to 500 kA. The length of the periods was 8hours.

The tests related to different types of pots. Table I contains thecharacteristics of some of the electrolytic cells placed under test andthe typical results obtained. In case A, the pots were regulated usingthe embodiment of the invention wherein Q(p) was determined using theterms Qint(p), Qc1(p), Qc2(p) and Qt(p). In case B, the pots wereregulated using the embodiment of the invention wherein Q(p) wasdetermined using the terms Qint(p), Qc1(p), Qc2(p), Qt(p) and Qe(p). Incase C, the pots were regulated using the embodiment of the inventionwherein Q(p) was determined using the terms Qint(p), Qc1(p), Qc2(p),Qt(p), Qr(p) and Qs(p).

The results show that the regulation method according to the inventionmakes it possible to regulate electrolytic cells effectively wherein theexcess AlF₃ of the bath is grater than 11% and wherein the bathtemperature is in the vicinity of 960° C. The preferred alternativeembodiments of the invention make it possible to regulate effectively,and with a surprising stability, electrolytic cells wherein theintensity and anode density are very high and wherein the liquid bathmass is low.

TABLE 1 Case A Case B Case C Intensity (kA) 300 kA 330 kA 500 kA Anodedensity (A/cm²) 0.78 0.85 0.90 Liquid bath mass (kg/kA) 25 22 17 ExcessAlF₃ (%) 11.8 11.8 13.2 Total standard deviation (σ %) 1.5 1.3 1.3Dispersion of excess AlF₃ at ±2 σ %  8.8–14.8  9.2–14.4 10.6–15.8 Bathtemperature (° C.) 962 962 961 Total standard deviation (σ %) 6 6 3.5Dispersion of temperature at ±2 σ % 950–974 950–974 954–968 Currentefficiency (%) 95.0 95.0 95.5

The applicant observed during its tests that the regulation methodaccording to the invention makes it possible to control, with highstability, the AlF₃ content of electrolytic cells, over a period ofseveral months, without having to take into account measured AlF₃contents, said measured contents are, in any case, easily affected bysignificant errors.

Advantages of the Invention

The method according to the invention makes it possible to account notonly for the average composition of the electrolyte bath of anelectrolytic cell, but also the impact of the solidified bath ridges onthis composition, said ridges, by eroding or growing, affect the bathcomposition.

1. Regulation method for an electrolytic cell for the production ofaluminium by means of electrolytic reduction of alumina dissolved in anelectrolyte bath based on cryolite, said cell comprising a pot, anodesand cathode components capable of circulating a so-called electrolyticcurrent in said bath, the aluminium produced by means of said reductionforming a pad referred to as a “liquid metal pad” on said cathodecomponents, said method comprising the supply of said cell with aluminain said bath and wherein it comprises: the set-up of a regulationsequence comprising a series of time intervals p of a duration Lpreferred to as “periods”; the determination of a mean temperature T(p)of the electrolyte bath, from at least one measurement of thetemperature of said bath made during the last period or at least one ofthe last Nt periods; the determination of a so-called “equivalent”quantity Qc1 (p) of AlF₃ contained in the alumina added to the cellduring the period p; the determination of a value Qm(p) of the totalequivalent AlF₃ supplies per period during the last period or during thelast N periods; the determination of a quantity Q(p) of aluminiumtrifluoride (AlF₃) to be added during the period p, referred to as“determined quantity Q(p)”, using the formula:Q(p)=Qint(p)−Qc1(p)+Qt(p), whereQint(p)=α×Qm(p)+(1−α)×Qint(p−1), α is a smoothing coefficient, Qt(p) isa determined function of the difference between said temperature T(p)and a set-point temperature To, the addition in said electrolyte bath,during the period p, of an effective quantity of aluminium fluoride(AlF₃) equal to said determined quantity Q(p).
 2. Regulation methodaccording to claim 1, wherein the calculation formula of the quantityQ(p) comprises an additional term Qc2(p), i.e.Q(p)=Qint(p)−Qc1(p)+Qt(p)+Qc2(p), where Qc2(p) is a corrective termwhich is a determined function of the difference between Qm(p) andQint(p).
 3. Regulation method according to claim 1, wherein said lengthLp of said periods is approximately the same for all the periods. 4.Regulation method according to claim 1, wherein said length Lp of saidperiods is between 1 and 100 hours.
 5. Regulation method according toclaim 1, wherein the term Qm(p) is calculated using the equationQm(p)=<Q(p)>+<Qc1(p)>, where: <Q(p)>=Q(p−1) and <Qc1(p)>=Qc1(p−1) whenthe term Qm(p) is determined using the total equivalent AlF₃ suppliesduring the last period; <Q(p)>=(Q(p−N)+Q(p−N+1)+ . . . +Q(p−1))/N, and<Qc1(p)>=(Qc1(p−N)+Qc1(p−N+1)+ . . . +Qc1(p−1))/N, when the term Qm(p)is determined using total equivalent AlF₃ supplies during the last Nperiods.
 6. Regulation method according to claim 5, wherein N is between2 and
 100. 7. Regulation method according to claim 1, wherein thecoefficient α is equal to Lp/Pc, where Pc is between 400 and 8000 hours.8. Method according to claim 1, wherein it comprises: the determinationof a quantity Qtheo corresponding to the total theoretical AlF₃requirements of the cell when regulation is started; the start-up of themethod by taking Qint(0)=Qtheo.
 9. Regulation method according to claim1, wherein the term Qt(p) is given by the equation Qt(p)=Kt×(Tp−To),where Kt is a constant.
 10. Regulation method according to claim 9,wherein Kt is between 0.01 and 1 kg/hour/° C.
 11. Regulation methodaccording to claim 1, wherein the term Qt(p) is limited by a minimumvalue and by a maximum value.
 12. Regulation method according to claim1, wherein the term Qc2(p) is given by the equationQc2(p)=Ko2×(Qm(p)−Qint(p)), where Ko2 is a constant.
 13. Regulationmethod according to claim 12, wherein Ko2 is between −0.1 and −1. 14.Regulation method according to claim 1, wherein the term Qc2(p) isoptionally limited by a minimum value and by a maximum value. 15.Regulation method according to claim 1, wherein, when the electrolyticcell comprises a mobile anode frame to which said anodes are attached,the quantity Q(p) comprises an additional term Qr(p) which is adetermined function of a quantity referred to as “specific resistancevariation” ΔRS which is equal to ΔR/ΔH, where ΔR is the variation of theresistance R of the cell measured when said frame is moved by adetermined distance ΔH, either upwards, ΔH being positive, or downwards,ΔH being negative.
 16. Regulation method according to claim 15, whereinthe term Qr(p) is given by the equation Qr(p)=Kr×(ΔRS−ΔRSo), where Kr isa constant and ΔRSo is a reference value.
 17. Regulation methodaccording to claim 16, wherein Kr is between −0.01 and −10kg/hour/nΩ/mm.
 18. Regulation method according to claim 15, wherein theterm Qr(p) is optionally limited by a minimum value and by a maximumvalue.
 19. Regulation method according to claim 1, wherein the quantityQ(p) comprises an additional term Qs(p) which is given by a determinedfunction of the difference between the surface area S(p) of said liquidmetal pad (12) and a set-point value So.
 20. Regulation method accordingto claim 19, wherein the term Qs(p) is given by the equationQs(p)=Ks×(S(p)−So), where Ks is a constant.
 21. Regulation methodaccording to claim 20, wherein Ks is between 0.0001 and 0.1 kg/hour/dm².22. Regulation method according to claim 19, wherein the term Qs(p) ispreferentially optionally limited by a minimum value and by a maximumvalue.
 23. Regulation method according to claim 1, wherein the quantityQ(p) comprises an additional term Qe(p) given by a determined functionof the difference between the excess AlF₃ measured E(p) and its targetvalue Eo.
 24. Regulation method according to claim 23, wherein the termQe(p) is given by the equation Qe(p)=Ke×(E(p)−Eo), where Ke is aconstant.
 25. Regulation method according to claim 24, wherein Ke isbetween −0.05 and −5 kg/hour/% AlF₃.
 26. Regulation method according toclaim 23, wherein the term Qe(p) is optionally limited by a minimumvalue and by a maximum value.
 27. Regulation method according to claims1, wherein the quantity Q(p) comprises an additional term Qea(p) whichis given by a determined function of the anode effect energy AEE. 28.Regulation method according to claim 27, wherein the term Qea(p) isoptionally limited by a minimum value and by a maximum value. 29.Regulation method according to claim 1, wherein the quantity Q(p) islimited to a maximum value Qmax.
 30. Regulation method according toclaim 1, wherein, when the determined value of the term Q(p) isnegative, its value is taken to be equal to zero, i.e. AlF₃ is not addedduring the period p.